1. Field of the Invention
The present invention relates, generally, to photovoltaic solar cells and, more particularly, to multijunction tandem photovoltaic solar cells. Specifically, the present invention relates to improved monolithic tandem photovoltaic solar cells which are efficient, radiation-resistant, and useful in space applications in addition to terrestrial applications.
2. Description of the Prior Art
Photovoltaic cells, commonly known as solar cells, essentially comprise semiconductors that have the capability of converting electromagnetic energy (such as light or solar radiation) directly to electricity. Such semiconductors are usually characterized by solid crystalline structures that have energy bandgaps between their valence electron bands and their conduction electron bands. When light is absorbed by the material, electrons that occupy low-energy states become excited to cross the bandgap to higher energy states. For example, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of the solar radiation, they can jump the bandgap to the higher energy conduction band.
Electrons which are excited to higher energy states leave behind unoccupied low-energy positions or holes. Such holes can shift from atom to atom in the crystal lattice and thereby act as charge carriers, as do free electrons in the conduction band, and contribute to the crystal's conductivity. Most of the photons absorbed in the semiconductor give rise to such electron-hole pairs which generate the photocurrent and, in turn, the photovoltage exhibited by the solar cell.
As is known, the semiconductor is doped with a dissimilar material to produce a space charge layer which separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage. If these hole and charge carriers are allowed to flow through an external load, they constitute a photocurrent.
It is known that photon energies in excess of the threshold energy gap or bandgap between the valence and conduction bands are usually dissipated as heat; thus they are wasted and do no useful work. More specifically, there is a fixed quantum of potential energy difference across the bandgap in the semiconductor. For an electron in the lower energy valence band to be excited to jump the bandgap to the higher energy conduction band, it has to absorb a sufficient quantum of energy, usually from an absorbed photon, with a value at least equal to the potential energy difference across the bandgap.
The semiconductor is transparent to radiation, with photon energies less than the bandgap. On the other hand, if the electron absorbs more than the threshold quantum of energy, e.g., from a higher energy photon, it can jump the bandgap. The excess of such absorbed energy over the threshold quantum required for the electron to jump the bandgap results in an electron that is higher in energy than most of the other electrons in the conduction band. The excess energy is eventually lost in the form of heat. The net result is that the effective photovoltage of a single bandgap semiconductor is limited by the bandgap.
Thus, in a single semiconductor solar cell, to capture as many photons as possible from the spectrum of solar radiation, the semiconductor must have a small bandgap so that even photons having lower energies can excite electrons to jump the bandgap. This, of course, involves attendant limitations. First, use of a small bandgap material results in a low photovoltage for the device and, naturally, lower power output occurs. Second, the photons from higher energy radiation produce excess energy which is lost as heat.
On the other hand, if the semiconductor is designed with a larger bandgap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the photons with lower energies will not be absorbed. Consequently, in designing conventional single-junction solar cells, it is necessary to balance these considerations and try to design a semiconductor with an optimum bandgap, realizing that in the balance there has to be a significant loss of energy from both large and small energy photons.
Much work has been done in recent years to solve this problem by fabricating tandem or multijunction (cascade) solar cell structures in which a top cell has a larger bandgap and absorbs the higher energy photons, while the lower energy photons pass through the top cell into lower or bottom cells that have smaller bandgaps to absorb lower energy radiation.
The bandgaps are ordered from highest to lowest, top to bottom, to achieve an optical cascading effect. In principle, an arbitrary number of subcells can be stacked in such a manner; however, the practical limit is usually considered to be two or three. Multijunction solar cells are capable of achieving higher conversion efficiencies since each subcell converts solar energy to electrical energy over a small photon wavelength band, over which it converts energy efficiently.
Various electrical connectivity options between subcells are possible, including: (1) series connected, (2) voltage matched, and (3) independently connected. In the series connected type of tandem solar cells, there is current matching of the two subcells. The advantage of the independently connected type is that it avoids the problems of having to electrically connect the two subcells. This type also allows more possibilities in designing the solar cell. However, it is more complex with respect to fabrication of the solar cell, and it is also more complex in terms of delivering the power from each separate cell to a single electrical load. This is a systems problem.
Such tandem cells can be fabricated in two different manners. The first manner involves separately manufacturing each solar cell (with different bandgaps) and then stacking the cells mechanically in optical series by any of a number of known methods. The disadvantage of this method is due to the complexity in forming such a stacked arrangement. The advantage is the flexibility of being able to stack different materials on top of each other.
The second manner of fabricating a tandem solar cell involves forming a monolithic crystalline stack of materials with the desired bandgaps. The advantage of this method is the simplicity in processing. The disadvantage is that there are a limited number of materials combinations which can be epitaxially grown in device-quality form.
It has been generally accepted by persons skilled in the art that the desired configuration for monolithic multijunction tandem devices is best achieved by lattice matching the top cell material to the bottom cell material. Mismatches in the lattice constants create defects or dislocations in the crystal lattice where recombination centers can occur to cause the loss of photogenerated minority carriers, thus significantly degrading the photovoltaic quality of the device. More specifically, such effects will decrease the open-circuit voltage (V.sub.oc), short circuit current (J.sub.sc), and fill factor (FF), which represents the relationship or balance between current and voltage for effective power output. Thus, the lattice-matched monolithic approach provides an elegant manner for the construction of a high-quality tandem cell.
One common problem with conventional semiconductors is their lack of radiation resistance, as would be required to ensure degradation-free operation in space. This problem is especially troublesome when considering space photovoltaics, where conventional Si solar cells degrade with time. Thus, alternate semiconductor materials have been investigated to overcome these problems.
Indium phosphide (InP) is an attractive III-V semiconductor for a variety of electronic device applications involving heterostructures because of the large number of lattice-matched III-V ternary and quaternary materials available, for example, GaAsSb, GaInAs, AlAsSb, GaInAsP, and AlInAs. In addition to being lattice matched, these compounds offer a wide range of bandgaps which aid in the design of complex device structures. InP is also considered a prime candidate for space photovoltaic applications because of its superior radiation hardness and demonstrated high efficiencies.
Thus, the possibility of constructing radiation-hard InP-based tandem solar cells for space application appears feasible. However, none has been disclosed in the art to date.
Other techniques for making a tandem solar cell are known. For example, U.S. Pat. 4,289,920 describes a two-cell cell construction in which different semiconductor materials are grown on opposite surfaces of a transparent insulating substrate. In other words, the two semiconductors are not in physical contact with each other. Consequently, there is no need to lattice match the two semiconductors; however, the problems associated with forming high-quality semiconductor layers on the intermediate substrate are undoubtedly substantial. A metal layer covering the bottom surface of the lower semiconductor reflects light through the structure. The metal layer wraps around the edge to connect the two cells in electrical series.
There has not heretofore been provided a monolithic tandem photovoltaic solar cell having the advantages and desirable combination of features which are exhibited by the devices of the present invention.